Insert into mysql db using c#.net

In my code shows error message like,

Error:

MySql.Data.MySqlClient.MySqlException (0x80004005): Column 'contactno' cannot be null

at MySql.Data.MySqlClient.MySqlStream.ReadPacket()
at MySql.Data.MySqlClient.NativeDriver.GetResult(Int32& affectedRow, Int64& insertedId)
at MySql.Data.MySqlClient.Driver.GetResult(Int32 statementId, Int32& affectedRows, Int64& insertedId)
at MySql.Data.MySqlClient.Driver.NextResult(Int32 statementId, Boolean force) at MySql.Data.MySqlClient.MySqlDataReader.NextResult() at MySql.Data.MySqlClient.MySqlCommand.ExecuteReader(CommandBehavior behavior) at MySql.Data.MySqlClient.MySqlCommand.ExecuteReader() at MySql.Data.MySqlClient.MySqlCommand.ExecuteNonQuery() at registration.Button1_Click(Object sender, EventArgs e) in c:\Users\elcot\Documents\Visual Studio 2010\WebSites\studentstafff\sfs\admin\registration.aspx.cs:line 86

and c# code for insert

protected void Button1_Click(object sender, EventArgs e)
{
        if (temp == 0)
        {
            try
            {
                //Guid newGUID = Guid.NewGuid();
                MySqlConnection conn = new MySqlConnection(ConfigurationManager.ConnectionStrings["ApplicationServices"].ConnectionString);
                conn.Open();
                string insertquery = "INSERT INTO students(name,email,password,course_id,contactno,doj,designation) VALUES(@username,@email,@password,@course,@mobileno,@dateofjoining,@designation)";
                MySqlCommand com = new MySqlCommand(insertquery, conn);

                //com.Parameters.AddWithValue("@ID", newGUID.ToString());
                com.Parameters.AddWithValue("@username", username.Text);
                com.Parameters.AddWithValue("@email", email.Text);
                com.Parameters.AddWithValue("@password", CreateMD5(password.Text));
                //com.Parameters.AddWithValue("@confirmpassword", Confirm Password.Text);
                com.Parameters.AddWithValue("@course", DropDownListcourse.SelectedItem.Value);
                com.Parameters.AddWithValue("@mobileno ", mobileno.Text);
                com.Parameters.AddWithValue("@dateofjoining", dateofjoining.Text);
                com.Parameters.AddWithValue("@designation", DropDownListDesignation.SelectedItem.Value);
                //execute 
                com.ExecuteNonQuery();

                //redirect to otherpage 
                //Response.Redirect("~/admin/admin.aspx");

                //some message
                Response.Write("Registration Successful");
                conn.Close();
                //Response.Write(" You registration is sucessfully completed!!!");

            }
            catch (Exception ex)
            {
                Response.Write("Error:" + ex.ToString());
            }
        }
    }

what is the problem in this code?